As noted in co-pending application Ser. No. 11/034,592, filed on even date herewith, by Taylor, et al. entitled “Method and Apparatus for Transmission Line and Waveguide Testing,” assigned to the assignee hereof and incorporated herein by reference, it was found that a multi-port junction in the form of an RF circuit could be used to detect distance to faults and the severity of the faults by coupling selected outputs of the multi-port junction to an Inverse Fourier Transform, which converts frequency domain information to time domain information, thus to be able to determine the distance to a fault by a peak in the output of the transform output.
As detailed in the above patent application, assuming that one can generate an estimate of the complex reflection coefficient from the output of a multi-port junction, then several techniques are detailed that improve the accuracy of the calculated distance to fault and severity of fault. These include taking into account attenuation per unit length of transmission line and determining the effect of prior faults on the fault in question to eliminate the effects of multiple faults. Moreover, the ability to estimate a complex reflection coefficient, rather than using an absolute magnitude, results in the ability to reduce or eliminate so-called ghosts while at the same time permitting a ready calibration procedure that eliminates cumbersome in-field calibration.
The ability to accurately locate faults in a line is an extremely important capability that greatly reduces repair time in any application that requires the use of transmission lines or waveguides, particularly in military aircraft. These aircraft contain many different types and lengths of transmission lines and waveguides that are wired throughout the body. The performance of these lines is essential to the operation of the aircraft and the safety of the crew. Due to the complexity of the wiring, the ability to pinpoint the distance to the faults in the line, and subsequently, the exact location of the corresponding defected sections of cable in the aircraft, greatly reduce repair time. Based on the number of military aircraft currently in service, this reduction in repair time results in a great cost savings for the military and also means that more aircraft can be ready for deployment sooner, once a fault has been determined.
This device can also be used to measure the distance to faults in lines that are used in commercial applications. One commercial example is Community Antenna Television (CATV) distribution systems that route television signals over cable to multiple subscribers. Being able to easily and accurately locate faults in these types of transmission lines could greatly reduce repair time.
Some commercially available fault location systems are based on frequency domain reflectometry. These devices use sophisticated, expensive electronics such as network analyzers to measure the complex reflection coefficients of a transmission line over a set frequency range. They then use this complex frequency response to determine the locations of each fault in the line. Because of the need of an expensive network analyzer, these devices are bulky and very expensive.
The above-mentioned patent application discloses a system whereby an inexpensive multi-port junction can be used in conjunction with an Inverse Fourier Transform to replace network analysis. There, the multi-port junction used was a six-port junction, which involved analysis of the power available at four output ports. The power available at these four ports was used to estimate the aforementioned complex reflection coefficient. In so doing, it became evident that since, for instance, for a 50-MHz-to-18-GHz frequency domain reflectometer, one needed to break up or divide the frequency bands into five bands, then one required a separate six-port junction and separate power detectors for each of the five bands. Because of the number of components required for the five six-port junctions and the four power detectors per junction, such a reflectometer required 20 power detection circuits. Moreover, the six-port junction required five passive elements, namely four 90° quadrature hybrids and one power divider. In short, one needed 45 elements.
For a five-band instrument such as described above, size and cost is a large factor, and so while it was found that with a six-port junction one could obtain the requisite accuracy, it was important to be able to reduce the size and complexity as well as cost of such a multi-band reflectometer.
Noting that if one could reduce the number of components, one could reduce the size of the reflectometer, a large factor especially in military applications. Moreover, one could reduce production cost if one could obtain like results with fewer components. Also, with fewer components one could materially reduce material cost, which in turn reduces manufacturing cost as well as testing and analysis costs. Further, with a system with reduced numbers of components, the system itself is easier to repair and with fewer components one has fewer potential sources of error that could affect the product. Moreover, with fewer components there are fewer replacement parts later in the life cycle of the product.
Thus, for a five-band reflectometer going from 0.5–1 GHz, 1–2 GHz, 2-4 GHz, 4–8 GHz and 8–18 GHz, in order to cover the entire frequency region of interest requires five unique circuits are required that are designed and tuned to these different frequency ranges. It is noted that the multi-port junctions and the associated passive microwave components have a limited frequency band, with the bandwidth of the components being usually on the order of an octave. That is the reason that the operating range from 0.5 GHz to 18 GHz is divided up into five bands on the order of an octave.
It should be also noted that power detection circuits of reasonable cost are band-limited. Therefore it is impractical to switch one set of power detection circuits between the various frequency ranges. One can purchase broadband power detectors, but since the power detectors themselves are approximately 80% of the cost of the frequency domain reflectometer, any way to reduce the number of power detectors is advantageous.
If one were tempted to use a broadband power detector, regardless of cost, one would nonetheless require switching devices between the power detectors and each of the five multi-port junctions. The use of such a switching device increases size and degrades performance because as soon as one interposes a component between the power detector and the circuit, one has to calibrate out the characteristics of the component, namely the switching component. Thus the interposition of these types of switches adds to the complexity of the calibration and the modeling necessary in order to properly calibrate such a reflectometer.
Even if the effect of the interposition of the switches is calibrated out, the interposition of the switches would degrade the performance because there are losses in the switches such that some energy coming from the multi-port junction that is supposed to go directly into the power detector gets reflected and causes standing waves inside the multi-port junction.
The result is that trying to multiplex power detectors and multi-port junctions is complex, expensive and impractical.
There is therefore a need to provide a less complicated and less costly reflectometer.